Floating flooring systems have been increasing considerably in popularity in recent years. Unlike nailed or glued flooring systems, the laying of which is difficult and has to be left to specialists, a particular advantage of floating flooring systems is their ease of installation (on an existing floor covering of the carpet, ceramic tile or vinyl type or directly onto a stone floor).
As is known, flooring strips are conventionally provided on their edges with tongue and groove coupling elements which are intended to join two adjacent strips together by interlocking during assembly of the flooring system.
Among the various systems which have been proposed for interlocking flooring strips intended for the production of floating flooring, those which have met with the greatest success among consumers and have consequently become so prevalent that they will doubtless shortly displace all other systems, are those known as “angular interlocking” systems. In these angular interlocking systems, two flooring strips are joined together by engaging the strip to be assembled at a certain angle (generally 45°) relative to the strip which has already been laid and then imparting to this strip a rotational movement having an axis which approximately coincides with the edges, which are in contact, of the tops of the strips, to bring it into the assembly plane.
Such a system is described, for example, in U.S. Pat. No. 4,426,820 granted to Heinz Terbrack. The flooring strips are rectangular in shape and are designed to be assembled by interlocking their opposing lengthwise edges. In order to do this, one edge is provided with a first coupling element comprising a groove and the other with a second coupling element comprising a tongue which is intended to cooperate with the first coupling element of an identical flooring strip. Mechanical locking means are incorporated into said coupling elements so as, when two flooring strips are mated in the assembly plane, to prevent these flooring strips from moving apart from one another in a direction perpendicular to that of said edges and parallel to the assembly plane. According to the coupling profile presented in U.S. Pat. No. 4,426,820, the groove of the first coupling element is defined by an upper lip and a lower lip. The lower lip extends beyond the upper lip and comprises a projecting locking element. The second coupling means comprises, in addition to the tongue which engages in the groove of the first coupling means, a locking channel which cooperates with the projecting locking element in order to immobilise the strip in the transverse direction.
Many improvements have since been suggested by manufacturers, the most useful of which is commonly known as the “click” system. The “click” system describes partial immobilisation of the strips relative to one another after assembly which is intended to hold them firmly together by preventing rotation in the uncoupling direction: if an attempt is made to detach the most recently inserted strip, it will only be detached if a certain force is applied. This partial locking has 2 advantages:                It facilitates the actual laying. This is because, before assembling the strips of the most recent row by their large sides with the flooring being laid, it is convenient to assemble them to one another by their small sides (which have identical profiles to the large sides) and to hold them in place with wedges. The “click” effect acting on the small sides makes it possible to fasten together the row of strips, to limit the number of wedges to be used and so facilitate insertion of the complete row.        It allows the user to be sure that all the strips are perfectly engaged in one another since they become firmly connected as a result of the “click”. In this way, it acts to a certain extent as an assembly indicator. This function is of genuine importance, essentially in the case of laminated products based on MDF (medium density fibreboard) or HDF (high density fibreboard) of low thickness (e.g. 6 mm) where the grooving is shallow and the projecting parts fragile.        
This partial immobilisation effect is achieved by generating friction, and more specifically by exploiting the elasticity of the material at the end of rotation during assembly of the boards.
Such a click system is described, for example, in patent EP 1 026 341 B1 held by UNILIN. In this system, the coupling elements are equipped with means which, once inserted into one another in two adjacent flooring strips, exert a mutual tension force which forces the strips towards one another. This is achieved by a particular configuration of the locking channel and the locking projection, which, in the assembled state, brings about an elastic deformation of the lower lip of the first coupling element and so generates the desired tension force.
Thus, in known systems, the “click” effect is typically the result of loading the intrinsic elasticity of the constituent materials of the strips. Furthermore, the forces in play during the click action will place a greater or lesser load on the strip at the point where it is generally at its thinnest.
Finally, it should be noted, and this is quite obvious from the interest aroused by this system, that this effect is all the more worthwhile the more marked it is.
It may therefore readily be understood that, in the case of thin products, for example of the HDF type, it becomes tricky to reconcile the strength of the material with the intensity of the “click” effect. This is because, for such materials, the thickness of the strip may be less than one mm.